INTEGRATED DIGITAL STATE GEOLOGIC MAP COMPILATION FOR THE UNITED STATES

The USGS Mineral Resources Program is completing a national digital geologic map database, utilizing state geologic maps, to support mineral resource studies at scales between 1:250,000 and 1:1,000,000. Digital versions of all existing state geologic maps were acquired. Where digital state geologic maps were lacking, new digital maps were prepared by digitizing existing printed maps either at the USGS or in cooperation with the respective state geologic survey. A uniform database structure was developed with consistent identification of feature types. Data from each state map database were imported into the uniform structure. All state spatial data were fit to a single standardized state boundary file (derived from the USGS 100K scale Digital Line Graphics boundary layer), enabling creation of regional digital maps without slivers or overlaps at the state boundaries. Fitting was done by extending or clipping arcs to the state boundary. No “rubber sheeting” was used. No attempt was made to reconcile differences in mapped geology between contiguous states.

Typically state geologic maps contain more data than just arcs and polygons. Unit descriptions as well as age and bibliographic information present on the original source map were captured in a series of four additional tables, including stratigraphic units, age, lithology, and references. For some states, more current information was also captured. In the tables, several fields were added in which a consistent set of terms was used for age and rock type, so that multiple spatial databases could be queried at the same time, allowing generation of regional and national derivative maps based on age and lithology. The minimum standard file set prepared for each state consists of a geology (polygon) database, metadata, and the four additional attribute tables; however, additional spatial databases for other line or point features present on the source map may also be included (e.g. faults, dikes, fold axes, volcanic vents).

Standardized state geological map databases and their related tables are available on-line as USGS Open-File Reports for the conterminous states. For Alaska and Hawaii, new state maps are being prepared and the preliminary work for Alaska is being released in a series of 1:500,000-scale regional compilations as USGS Open-File Reports.